Project 2 lead by Victor Nizet, M.D. complements the overall PEG in a unique fashion, by probing what happens when the glycobiological homeostasis of the mammalian host is perturbed by exogenous factors. Specifically, this project examines the innate immune and inflammatory functions of myeloid cells (neutrophils and macrophages) upon challenge by infectious microbes that either (a) display glycans that mimic common host glycoconjugates or (b) produce glycosidases that can target (cleave) host glycoconjugates. Either phenotypic property has the potential to alter the glycobiologial homeostasis of the host and subvert normal myeloid cell signaling, innate immunity, and inflammatory responses. The project focuses upon three bacterial pathogens of great importance to human medicine, group A Streptococcus (GAS), group B Streptococcus (GBS) and Streptococcus pneumoniae (SPN). Our molecular genetic approach involves generation of precise, live isogenic bacterial reagents that differ only by the expression of a surface glycan or the deployment of a surface-associated or secreted glycosidases. Our PEG collaborators provide complementary expertise and reagents to measure changes in, or genetically alter, the corresponding glycan receptor molecules on host leukocytes or in the whole animal. In Aims 1 and 2, we use GBS to determine how bacterial sialic acid (Sia) mimicry and SPN sialidase expression modulate CD33rSiglec- mediated myeloid cell innate immune and inflammatory responses, respectively. In Aims 3 and 4, we determine how GAS hyaluronic acid (HA) expression modulates and how GBS hyaluronidase expression modulates CD44-mediated myeloid cell innate immune and inflammatory responses Deploying this unique suite of tools, in which the host-pathogen equation is carefully manipulated in a controlled fashion from both sides, we will study infectious disease pathogenesis and innate immune responses in ex vivo and in vivo models of myeloid cell innate immune and inflammatory function.